How Does the Mass Flow Hypothesis Explain Phloem Transport?
This theory was proposed by German plant physiologist Ernst Munch in 1930. It is stated by the mass flow hypothesis that the translocation of glucose and other sugars within phloem are caused by a never-ending flow of water and dissolved nutrients between the source (a place where sugars are made) and sink (where sugars are utilized). A diffusion gradient or osmotic gradient is caused due to the high concentration of sugar and other organic substances by the phloem source cells. This results in water being drawn out from adjacent xylem and hydrostatic pressure which moves the sap. Above stated theory is the closest explanation available to describe mass flow in the phloem.
Mechanism- How it Works
The mechanism of translocation is the movement of minerals and water through the xylem as a result of negative pressure and movement through the phloem as the result of hydrostatic pressure. This movement is accompanied by the process of phloem loading and unloading.
Description of the Mass Flow Hypothesis
The cells present in food sources load a sieve tube through osmotic potentials, pushing the sap lower. These cells then deliver the solutes out of the sieve-tube producing the unloading effect. When asked to explain the mass flow hypothesis in detail, this is the answer:
Plants respire through the process of photosynthesis, which involves the formation of glucose in mesophyll cells. Not all of the sugars are utilized; leftover glucose becomes non-reducing sugar.
Sugars like sucrose are then delivered to the neighbor cells present in the veins of leaves.
Through the plasmodesmata, the sugars diffuse from neighbor cells and enter the sieve tube. The number of sugars within the sieve tube and its elements increase. This is the beginning of the mass flow of the hypothesis mentioned.
At the same time, water travels from the nearby xylem to the source cells via osmosis to increase hydrostatic pressure in the elements of the sieve tube. This is the mass flow hypothesis for the mechanism of translocation in plants.
This hydrostatic pressure turgor then shifts the sugars and other substances down the cells of the sieve tube towards the sink (roots).
At the sink, the sugars are destroyed to become apoplast and then enter the symplast.
The water that traveled out of the cells reduces the hydrostatic pressure in them. The gradient created is the result of the pressure developed by the entry of sugar and the source and the elimination of it at the sink. Learning mass flow hypothesis at a level is more complex.
The root cortex and stem eliminate the phloem sugar and use it for cellular respiration. Starch does cause any pressure as it is insoluble. At the end through transpiration pull the pure water left is absorbed.
For mass flow hypothesis a level biology the concepts are more advanced and detailed. If you want to learn more, read high-level books, and converse with experts.
The Pressure Flow Hypothesis
The transport begins in sieve tubes at source regions (the place where photosynthesis takes place) after photosynthesis. The creation of food raises osmotic pressure within the sieve tube. This removes the sugars from sieve tubes towards the sink regions (the place where utilization of materials takes place) to lower the pressure. A potential is created from the source to the sink within the sieve tubes, in which the solution flows.
Criticisms of the Mass Flow Hypothesis
The majority of people that reject this hypothesis believe that mechanisms such as the mechanism of translocation in plants take place due to metabolic processes, not hydrostatic pressure. Critics believe that mass flow is mainly a passive process and companion cells support sieve tube vessels. This indicates that the hypothesis negates the phloem's living nature. The hypothesis also states that the rate of transport of materials is uniform which has been disproved. Amino acids and other nutrients like sugars are translocated at different rates throughout. Also, the Munch mass flow hypothesis only accounts for the unidirectional movement of pressure flow. It doesn't explain the bilateral movement phenomena of materials moving at the same time in opposite directions. If the hypothesis is accepted, then that would mean that the bidirectional movement isn't possible, which is false.
Benefits of the Mass Flow Hypothesis Article
If you are preparing for the NEET exam and are looking forward to cracking it on the first attempt, you may come across various topics and concepts which will be very hard to comprehend. One such topic is the Mass Flow Hypothesis. If you want to understand the terminologies, concepts used in this topic, you can refer to this article. In this article, we have covered:
Mass Flow Hypothesis states
who gave this hypothesis
what is the mechanism behind the translocation of sap
why was this theory criticized, etc
Via this article, you will be able to understand every term. This article will help you to clear your concept and if any question is asked from this topic, you will be easily able to answer that. Students who are unable to grasp some advanced terms should not worry, because Vedantu has got them covered. At Vedantu, they will be able to find different resources relevant to the Mass Flow Hypothesis. For example, they will find video lectures explaining this topic and in that video, everything is explained very deeply about the Mass Flow Hypothesis.
Mass Flow Hypothesis Theory
The mass flow hypothesis is a theory that tells about how the sap flows through the phloem. This theory or hypothesis was first proposed in 1930 by German physiologist Ernst Munch. This theory tells us how simple sugar or a sugar that is highly concentrated organic sugar, is transported to other parts of a plant. It is based on the principle of hydrostatic pressure. If we take an example of a leaf, the phloem from that leaf creates a diffusion gradient with the help of which, it can draw water from the adjacent xylem. In phloem, the movement of sugar and other materials occurs by mass flow. This movement is bidirectional which means the mass can flow in both directions. This multi-directional flow doesn't let the sap flow in the opposite directions. It is usually seen that the movement of materials via the phloem is driven by positive pressure (hydrostatic). The same movement through the xylem is because of negative pressure or tension. This theory states that Glucose and other sugars are translocated in the phloem. This movement is caused because of the pressure created by the flow of water and nutrients.
Mechanism Of Mass Flow Hypothesis
In the xylem, the flow of nutrients, minerals, and water are caused because of the creation of negative pressure, as discussed earlier but the movement via the phloem is caused by the positive pressure (hydrostatic) this also results in the loading and unloading of phloem.
Simple Explanation of the Munch Mass Flow Hypothesis
The leaves are the end, that is, the place where the food is synthesized and produced. The roots are the sink end, where the food is absorbed and utilized. In an experiment with two osmometers with different concentrations, it was seen that the theory is right about the movement of food from the leaves to the roots.
Fun Fact
The mass flow hypothesis mechanism was sourced from Ernst Munch’s book Die Stoffbewegungen in der Pflanze (translated to The Movements of Matter in Plants), with 230 pages dedicated to just fluid circulation within plants. Interestingly, Munch claimed that his concept was nothing novel. His famed colleagues had understood it before he did. No other botanist has formulated a hypothesis like Munch’s yet. This is the reason why his name is taken synonymously with this hypothesis.
FAQs on Mass Flow Hypothesis: Definition, Process & Importance
1. What is the Mass Flow Hypothesis in plant biology?
The Mass Flow Hypothesis, also known as the Pressure Flow Hypothesis, is the most accepted theory explaining how sugars (mainly sucrose) are transported through the phloem of a plant. It states that this movement occurs in bulk from an area of high sugar concentration and pressure (the source, like leaves) to an area of low concentration and pressure (the sink, like roots or fruits), driven by a hydrostatically generated pressure gradient.
2. Who first proposed the Mass Flow Hypothesis?
The Mass Flow Hypothesis was first proposed in 1930 by the German plant physiologist Ernst Münch. Because of his foundational work, the theory is sometimes referred to as the Münch hypothesis.
3. What are the key steps involved in the Mass Flow mechanism?
The mechanism of mass flow involves a sequence of interconnected steps:
- Phloem Loading: Sucrose from photosynthesis is actively transported into the sieve-tube elements in the leaves (source), requiring ATP.
- Water Entry: The high concentration of sugar lowers the water potential in the phloem, causing water to move in from the adjacent xylem via osmosis.
- Pressure Creation: This influx of water generates high hydrostatic pressure (turgor pressure) at the source end.
- Bulk Flow: The pressure gradient between the source and the sink pushes the phloem sap (sugar-rich solution) along the sieve tube.
- Phloem Unloading: At the sink, sucrose is actively transported out of the phloem for storage or use, again requiring ATP.
- Water Exit: As sugar leaves, the water potential in the phloem rises, and water moves back into the xylem, lowering the pressure at the sink.
4. What is the difference between a 'source' and a 'sink' in phloem transport?
In the context of phloem transport, a source is any part of the plant that produces or releases sugars, primarily mature photosynthetic leaves. A sink is any part of the plant that consumes or stores these sugars, such as roots, flowers, fruits, and growing points. The roles are dynamic; for example, a potato tuber acts as a sink when it forms but becomes a source when it sprouts the following spring.
5. Why is ATP required for transport if mass flow is driven by pressure?
This is a crucial concept. While the physical movement of sap through the sieve tube is a passive process driven by a pressure gradient, the establishment of this gradient is an active process that requires energy (ATP). ATP is essential for two key stages: actively loading sucrose into the phloem at the source and actively unloading it at the sink. Without this energy expenditure to create and maintain the concentration difference, the pressure gradient would not exist, and mass flow would stop.
6. What are the main criticisms or limitations of the Mass Flow Hypothesis?
Despite being widely accepted, the Mass Flow Hypothesis has some limitations. Critics point out that it does not fully explain certain observations, such as:
- The presence of sieve plates between phloem cells, which would seem to obstruct a simple bulk flow.
- Observations that different substances in the phloem can move at different rates, which contradicts the idea of a single mass flow.
- The bidirectional movement of solutes that is sometimes observed within a single sieve tube.
7. How does the girdling experiment provide evidence for the Mass Flow Hypothesis?
The girdling or ringing experiment offers strong support for the hypothesis. In this experiment, a ring of bark, which includes the phloem tissue, is carefully removed from a woody stem. After a period, the area above the ring swells up and is found to be rich in sugars. This occurs because the downward flow of sugars from the source (leaves) to the sink (roots) is interrupted at the ring. This accumulation demonstrates that food is transported downwards via the phloem, a central principle of the Mass Flow Hypothesis.
